Yup, @kpsns nailed it. LaTeXML does the heavy lifting in converting TeX to XML. From there some post processing does the job of converting it to a nice responsive template (that's done by Engrafo / the ArXiv Vanity team).
We love OSS at AI2, and are looking to collaborate with the Engrafo / ArXiv Vanity team as we expand the functionality.
We love OSS at AI2, and are looking to collaborate with the Engrafo / ArXiv Vanity team as we expand the functionality.